According to a conventionally known technique, direct current voltage is output after being decreased or increased by a switching power source circuit (see, e.g., Japanese Laid-Open Patent Publication No. H11-299224). For example, the value of voltage output from the switching power source circuit is maintained at a constant value by increasing or decreasing the ratio of the ON time period to the OFF time period of a switching device of the switching power source circuit.
In this configuration, a sub-harmonic oscillation may occur when the ON time period of the switching device is controlled by feeding back the current from the switching power source circuit. According to another known technique, when the current controlling the switching power source circuit from the power source circuit is fed back, sub-harmonic oscillation is prevented by adding a predetermined current to the current that is fed back (see, e.g., Japanese Laid-Open Patent publication Nos. 2011-91888 and 2011-101479).
Nonetheless, the occurrence of errors increases by the amount of current added to the output current supplied from the switching power source circuit consequent to variations such as manufacturing conditions, and device temperature and the voltages during use.